Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow for adsorption of molecules with certain dimensions while rejecting those of large dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves such as crystalline silicates, aluminosilicates, crystalline silicoalumino phosphates, and crystalline aluminophosphates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term “zeolites” includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms “zeolite”, “zeolites” and “zeolite material”, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term “aluminosilicate zeolite”, as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Within a pore of the zeolite, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, disproportionation, alkylation, and transalkylation of aromatics are governed by constraints imposed by the channel size of the zeolite. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the pores to react; while product selectivity occurs when some of the products can not leave the channels or do not subsequently react. Product distributions can also be altered by transition state selectivity in which certain reactions can not occur because the reaction transition state is too large to form within the pores. Shape selectivity can also result from configuration constraints on diffusion where the dimensions of the molecule approach that of the pore.
Non-selective reactions on the external surface acid sites of zeolites are usually not desirable because they are not subject to the shape selective constraints imposed on those reactions occurring within the channels of the zeolites and are often not desirable. For example, the surface acid sites can adversely effect reactants entering the pores of the zeolite and products exiting the pores of the zeolite.
Various procedures have been prepared for limiting surface acidity of the zeolites. For example, the surface acidity of aluminosilicate zeolites can be reduced by dealumination of the zeolite surface. Techniques for the dealumination include hydrothermal treatment, mineral acid treatment with HCl, HNO3, or H2SO4, and chemical treatment with SiCl4 and EDTA. A problem associated with these procedures is that they can degrade the zeolite crystals and/or cause the zeolite crystals to lose sorption capacity. Still further, dealumination can also reduce the overall activity of the zeolite, which can result in reduced catalyst activity and/or activity maintenance.
Another procedure for limiting the surface acidity of zeolites, as disclosed in U.S. Pat. No. 4,203,869, involves synthesizing an isocrystalline layer of aluminum-free zeolite over the surface of the zeolite.
The present invention is directed to coated zeolite catalysts and the use of the coated zeolite catalysts in hydrocarbon conversion processes.